1. Field of the Invention
The present invention relates to a base station and a receiver failure diagnosing method, and particularly to a base station and a receiver failure diagnosing method in a radio communication system.
2. Description of the Related Art
System stability is one of important elements to put a mobile communication system into practice. In order to operate the system stably, it is required not only to prevent occurrence of any failure causing stop of the system operation, but also to quickly detect and restore an occurring failure. Accordingly, a failure detecting circuit and a diagnosing system thereof in a radio base station are very important.
A transmitter and a receiver are mounted in a radio base station. Failure detection of the transmitter can be relatively easily implemented by branching a part of a transmission main signal generated by the transmitter and monitoring the part concerned. On the other hand, it is impossible to implement failure detection of the receiver by merely branching and monitoring a part of a reception signal. This is because the power of the reception signal input to the receiver varies every second in accordance with an installation environment and the connection number of terminals, etc. and thus it is impossible to set a threshold value for determining whether the reception power value is normal or abnormal. Accordingly, in order to implement the failure detection of the receiver, it is general to input some well-known test signal to the receiver and monitor the reception state of the receiver.
For example, JP-A-2002-246978 discloses a method of diagnosing a receiver by using a test signal whose power value is well known. According to this method, a test signal generator for outputting a test signal or a mechanism for distributing a transmission signal and then inputting the distributed transmission signal to a receiver is installed in the same radio base station. Furthermore, JP-A-2006-319616 discloses a simple receiver diagnosing method which does not use any well-known test signal, but use a receiver thermal noise.
The method disclosed in JP-A-2006-319616 uses, not a test signal, but thermal noise existing in the receiver for failure detection of the receiver. The thermal noise occurs because free electrons in a conductor make movements due to thermal energy, and it is uniformly distributed at all the frequencies. Accordingly, thermal noise necessarily occurs in a conductor existing at the input terminal of a receiver irrespective of the type of the receiver. Occurring thermal noise is amplified by an amplifier in the receiver as in the case of other reception signals, and then input to a demodulator. The failure detection circuit has, in the receiver, three high-frequency switches and an automatic gain control amplifier for making signal power input to the demodulator constant. By switching the three high-frequency switches, the signal passage in the receiver is switched, and normality of the receiver is diagnosed by using a gain value of the automatic gain control amplifier in each case.